I've been stuck fighting Undying all day long and I can't defeat her. It's either my tactic is bad or I don't have the right items..? I have Cloudy Glasses and Torn Notebook equipped currently and I have my inventory filled with healing items. But it's been a few weeks and I still can't defeat her. Any tips for me?
From the Undertale Wiki, you can flee from the battle to reset her attack speed once your SOUL mode has been changed to red. She will catch up with you again, but at least the subsequent attacks will be less vicious.
If you are trying to spare her for a pacifist run, you must continue to flee until you reach Hotland where she will collapse and the fight will be over. At which point
pouring water on her unlock a date with her, a requirement for reaching the true pacifist ending.
If you are going for a neutral route, you may switch between fleeing and attacking her as necessary until she is defeated, though
once she reaches 0 HP, she will cling on with determination and continue to fight for a few more turns. Though her attacks get slower every turn, your attacks will do nothing so you have to outlast her final assault.
Because of this and because the fight can be so difficult, it is probably better to simply flee the fight even if you do not wish to spare her.
If you are going for a genocide route, this fight cannot be avoided and is very difficult, so the only advice I can offer is to practice until you memorize the patterns of her attacks.